The Internet has dramatically increased the potential for data, voice, and video services for customers. Existing circuit-switched telephony systems, however, do not provide the foundation to support the growing need for bandwidth and new services required by both residential and business consumers. As a result, integrated access devices have been introduced to support Internet and related technologies as well as standard telephony service.
Integrated access devices often employ asynchronous transfer mode (ATM) functionality to multiplex data, voice, and video traffic together onto a single network. ATM is a connection-oriented packet-switching technology in which information is organized into small, fixed length cells. ATM carries data asynchronously, automatically assigning data cells to available time slots on demand to provide maximum throughput. Compared with other network technologies, ATM provides large increases in maximum support bandwidth, support for multiple types of traffic such as data, video, and voice transmissions on shared communication lines, and virtual networking capabilities, which increase bandwidth utilization and ease network administration.
ATM adapts different types of traffic to fit into the standardize ATM cell format. The standard ATM cell is 53 bytes in length and includes a 5 byte header followed by a 48 byte payload. For particular types of traffic, the payload includes a traffic-specific ATM adaptation layer (AAL) that allows the traffic to be transmitted in the ATM cell and reconstituted at the far end of the ATM network.
ATM adaption layer 1 (AAL1) is a cell format used by ATM to transport telephony traffic. Telephony traffic is carried in DS-0 channels that include an eight (8) bit voice sample transported through the network at regular 125 microsecond intervals. In the case of structured AAL1, a number of such DS-0 bytes are group together in the payload of an ATM cell and then carried through an ATM network in the ATM cell. The DS-0 bytes are fixed in alignment within the ATM cell payload to facilitate switching of the DS-0s.
One or more of the DS-0 channels may be sub-utilized in that it contains only a ¼ DS-0 or may be a combined DS-0 channel containing up to four ¼ DS-0s. This is the case for basic rate integrated services digital network (ISDN) traffic that includes two B-channels each comprising a DS-0 and a D-channel comprising a ¼ DS-0. Traffic for each basic rate ISDN connection may be transmitted in a 3 DS-0 format with one of the DS-0 channels being sub-utilized or in a 4:1 DS-0 format with the ¼ DS-0 for four (4) D-Channels being combined into a single DS-0.
In addition to a standard DS-0 switch, a supplemental ¼ DS-0 switch is used to switch ISDN and other types of traffic having sub DS-0 traffic. The supplemental ¼ DS-0 switch is used to expand, consolidate, and switch the ¼ DS-0s between DS-0 channels. Although the supplemental ¼ DS-0 switch requires additional circuitry that increases the cost of the switch core and takes up valuable space on the switch card, it is necessary because conventional ATM switches cannot address and switch sub DS-0 traffic.